1. Field of the Invention
This invention relates to a transmission circuit for use in a radio transmitter, such as a transmission circuit used in the field of mobile radio communication.
2. Description of the Related Art
FIG. 1 is a block diagram showing the structure of a prior art transmission circuit. Referring to FIG. 1, the prior art transmission circuit includes a power amplifier circuit 1, an output monitor circuit 2, an output detector circuit 3, an error detector circuit 4, a reference signal generator circuit 5, a high-frequency signal input terminal 6, a gain control terminal 7 and a transmission output terminal 8.
The operation of the prior art transmission circuit having the above structure will now be described. A high-frequency signal inputted through the high-frequency signal input terminal 6 is amplified by the power amplifier circuit 1, and, after being then branched or split by the output monitor circuit 2, part of the amplified high-frequency signal is outputted from the transmission output terminal 8. The remaining part of the amplified high-frequency signal is inputted to the output detector circuit 3, and the output signal V.sub.DET of the output detector circuit 3 is inputted to the error detector circuit 4. In the error detector circuit 4, the detected voltage signal V.sub.DET is compared with a reference signal V.sub.REF from the reference signal generator circuit 5 to detect the error voltage, and the detected error voltage signal is then inputted to the power amplifier circuit 1 through the gain control terminal 7 so as to control the gain of the power amplifier circuit 1.
However, in the case where the high-frequency signal inputted through the high-frequency input terminal 6 of the transmission circuit having the structure as described above is a linear modulated signal whose amplitude carries some kind of information, this manner of loop control will cancel the modulation when the reference signal V.sub.REF is a constant voltage. Thus, the prior art transmission circuit has had the problem that its output waveform tends to be distorted.